Question on superconductor behaviour [Archive]

Lord Snooty

Jun27-04, 06:02 PM

Igor Khavkine

Jun30-04, 05:36 PM

<jabberwocky><div class="vbmenu_control"><a href="jabberwocky:;" onClick="newWindow=window.open('','usenetCode','toolbar=no, location=no,scrollbars=yes,resizable=yes,status=no ,width=650,height=400'); newWindow.document.write('<HTML><HEAD><TITLE>Usenet ASCII</TITLE></HEAD><BODY topmargin=0 leftmargin=0 BGCOLOR=#F1F1F1><table border=0 width=625><td bgcolor=midnightblue><font color=#F1F1F1>This Usenet message\'s original ASCII form: </font></td></tr><tr><td width=449><br><br><font face=courier><UL><PRE>On Sun, 27 Jun 2004 23:02:09 +0000, Lord Snooty wrote:\n\n> Take a tube of Type II superconductor with a copper wire fed through its\n> centre. The wire carries a current that can be switched on or off.\n> Consider two scenarios:\n> A.\n> 1) The wire current is switched on\n> 2) Then the tube is cooled to below its critical temperature.\n>\n> B.\n> 1) The tube is cooled to below its critical temperature 2) Then the wire\n> current is switched on.\n>\n> My questions are, for both scenarios: 1) Does a superconducting current\n> flow in a thin layer on the inside surface of the tube (the\n> Meissner-Ochsenfeld effect)?\n\nYes for both cases. This page provides a nice explanation and illustration:\nhttp://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html#c2\n\n> 2) What happens to this current (if it\n> exists) when the wire current is switched off?\n\nThis current exists to block out the magnetic field from the bulk of the\nsuperconductor. If you turn off the magnetic field very slowly, the\nsurface current in the superconductor will decrease together with the\nmagnetic field. However, if the magnetic field is not turned off not so\nquickly, the generated electric field (Faraday\'s law) cannot be neglected\nand may generate a residual current in the superconductor. Since this\ncurrent will encounter no resistance in the superconductor, it will\npersist after the the magnetic field inside the cylinder is turned off,\nassuming that it has somewhere to flow.\n\nHope this helps.\n\nIgor\n\n</UL></PRE></font></td></tr></table></BODY><HTML>');"> <IMG SRC=/images/buttons/ip.gif BORDER=0 ALIGN=CENTER ALT="View this Usenet post in original ASCII form">  View this Usenet post in original ASCII form </a></div><P></jabberwocky>On Sun, 27 Jun 2004 23:02:09 +0000, Lord Snooty wrote:

> Take a tube of Type II superconductor with a copper wire fed through its
> centre. The wire carries a current that can be switched on or off.
> Consider two scenarios:
> A.
> 1) The wire current is switched on
> 2) Then the tube is cooled to below its critical temperature.
>
> B.
> 1) The tube is cooled to below its critical temperature 2) Then the wire
> current is switched on.
>
> My questions are, for both scenarios: 1) Does a superconducting current
> flow in a thin layer on the inside surface of the tube (the
> Meissner-Ochsenfeld effect)?

Yes for both cases. This page provides a nice explanation and illustration:
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/meis.html#c2

> 2) What happens to this current (if it
> exists) when the wire current is switched off?

This current exists to block out the magnetic field from the bulk of the
superconductor. If you turn off the magnetic field very slowly, the
surface current in the superconductor will decrease together with the
magnetic field. However, if the magnetic field is not turned off not so
quickly, the generated electric field (Faraday's law) cannot be neglected
and may generate a residual current in the superconductor. Since this
current will encounter no resistance in the superconductor, it will
persist after the the magnetic field inside the cylinder is turned off,
assuming that it has somewhere to flow.

Hope this helps.

Igor